NIA Pilot Research Program, Topic 14: Sensory and Motor Processing. Older adults exhibit a reduced ability to perform accurate goal-directed movements with various muscle groups. This impairment is often observed with movements of the index finger and attributed to a reduced capacity to exert consistent movement trajectories (Enoka et al. 2003). Recent findings however, provide evidence that the ability of older adults to match the target movement trajectory with the index finger is impaired despite lower fluctuations (more consistent) in the movement trajectory (Christou et al. 2003). Although the physiological mechanisms that contribute to movement accuracy are largely unexplored, the impaired accuracy in older adults may be due to: (1) inconsistent movement trajectory caused by an altered activation of the motor units in the agonist muscle (2) impaired acceleration of the index finger caused by altered co-activation of the agonist and antagonist muscles. Based on the preliminary findings, it is hypothesized that the reduced accuracy of older adults with goal-directed movements is due to altered levels of co-activation. Two specific aims will test this hypothesis: The first aim will determine the association between fluctuations in movement trajectory and the accuracy of a pointing task with the index finger during interventions that impair (movement speed) or enhance (practice) accuracy in young and older adults. The second aim will establish the contributions of motor unit discharge and co-activation of the agonist and antagonist muscles to the difference in the pointing accuracy. The findings of this RO3 application will constitute novel information on the physiological mechanisms that impair movement accuracy in older adults. These preliminary findings will form the foundation for an RO1 application that will further explore the mechanisms contributing to the impaired movement accuracy in older adults. [unreadable] [unreadable]